RESEARCH ARTICLE Late Miocene Intensified Tectonic Uplift and Climatic 10.1029/2018GC007917 Aridification on the Northeastern Tibetan Plateau: Key Points: • A long‐term gradual aridification Evidence From Clay Mineralogical and since 12.7 Ma and an enhanced drying event since ~7.8 Ma occurred Geochemical Records in the in the Xining Basin • Uplift of the northeastern Tibetan Xining Basin Plateau was the major driver of the Rongsheng Yang1,2,3 , Yibo Yang1,4 , Xiaomin Fang1,4,2 , Xiaobai Ruan1,2,3, Albert Galy3 , evolution of regional climate and 1 5 6 landscape since the late Miocene Chengcheng Ye , Qingquan Meng , and Wenxia Han • Clay minerals are well‐used 1 potential proxies for paleoclimatic Key Laboratory of Continental Collision and Plateau Uplift, Institute of Tibetan Plateau Research, Chinese Academy of reconstructions in complex Sciences, Beijing, China, 2College of Resources and Environment, University of Chinese Academy of Sciences, Beijing, sedimentary routing systems China, 3Centre de Recherches Pétrographiques et Géochimiques, UMR7358, CNRS and Université de Lorraine, Vandœuvre‐lès‐Nancy, France, 4CAS Center for Excellence in Tibetan Plateau Earth Sciences, Chinese Academy of Supporting Information: 5 • Sciences, Beijing, China, School of Earth Sciences and Key Laboratory of Western China's Mineral Resources of Gansu Supporting Information S1 6 • Data Set S1 Province, Lanzhou University, Lanzhou, China, College of Resources and Environment, Linyi University, Linyi, China Abstract The uplift of the Tibetan Plateau (TP) during the late Cenozoic is thought to be one of crucial Correspondence to: X. Fang, factors controlling Asian climate. However, the complex interaction between tectonics and climate change [email protected] is still unclear. Here we present the first record of clay mineralogy and elemental geochemistry covering ~12.7–4.8 Ma in a fluvial‐lacustrine sequence in the Xining Basin. Geochemical provenance proxies (Th/Sc, ‐μ fi ‐ Citation: Zr/Sc, and Cr/Zr) in the <2 m fraction show a signi cant provenance change at ~8.8 Ma. Silicate based Yang, R., Yang, Y., Fang, X., Ruan, X., weathering indexes (CIA, CIW, and PIA) displayed coeval changes with provenance but discrepant changes Galy, A., Ye, C., Meng, Q., & Han, W. with regional climate. Since the clay mineralogy exhibits significant change at ~7.8 Ma uncorrelated with fi (2019). Late Miocene intensi ed fi tectonic uplift and climatic aridification modi cations in provenance, it can be employed to reveal regional climate change. The rise in illite and on the northeastern Tibetan Plateau: associated decrease in the sum of smectite and illite/smectite mixed layers reflect gradual and slow Evidence from clay mineralogical and aridification since ~12.7 Ma with intensified drying since ~7.8 Ma until approaching the modern climate geochemical records in the Xining Basin. Geochemistry, Geophysics, status. Our results, together with other regional climatic and tectonic records, clearly illustrate that Geosystems, 20, 829–851. https://doi. accelerated uplift of the northeastern TP since ~8–9 Ma has mainly modulated the regional erosion, org/10.1029/2018GC007917 weathering, transportation, and sedimentation and amplified the global cooling and drying trend toward the regional climate of modern conditions. Our study suggests that in the tectonically active northeastern TP, a Received 20 AUG 2018 fi ‐ Accepted 14 JAN 2019 comprehensive mineralogical and geochemical investigation of the ne grained fraction of the basin Accepted article online 17 JAN 2019 sediments could retrieve the interactions between tectonics and climate behind the complex change in Published online 7 FEB 2019 exhumed lithology and sedimentary routing systems. 1. Introduction The paleoenvironment and paleoclimate of Asia dramatically changed during the late Cenozoic, such as enhanced central Asian aridification (Caves et al., 2016; Ding et al., 2005; Guo et al., 2002; X. D. Liu et al., 2015; Lu & Guo, 2014; Ruddiman & Kutzbach, 1989) and East Asian monsoon development (An et al., 2001; Guo et al., 2002). Generally, uplift of the Tibetan Plateau (TP) could lead to those transitions of paleoenvironmental and paleoclimatic patterns in Asia by blocking moisture from the ocean, reorganizing Asian atmospheric circulation, and strengthening the heating difference between land and ocean (Kutzbach et al., 1989; Li & Fang, 1999; X. D. Liu & Yin, 2002; Manabe & Terpstra, 1974; Tada et al., 2016). Therefore, the uplift of the TP in the late Cenozoic is considered one of the major drivers forcing the Asian paleoenvironmental changes, as evidenced from paleoreconstructions (An et al., 2001; Li et al., 2014; Ma et al., 1998; Xiao et al., 2012; Y. B. Yang et al., 2016) and modeling work (Kutzbach et al., 1989; X. D. Liu & Yin, 2002; X. D. Liu et al., 2015; R. Zhang, Jiang, et al., 2015). However, the complex interaction between tectonic and climate change is still unclear. This is largely due to (1) the combined effect of aridifi- ©2019. American Geophysical Union. cation of the continent by oceanic closure and uplift of the plateau and (2) the lack of temporally and spa- All Rights Reserved. tially matching tectonic and climatic records in a critical zone. The northeastern TP and its adjacent YANG ET AL. 829 Geochemistry, Geophysics, Geosystems 10.1029/2018GC007917 central Asia are thought to be ideal regions for revealing the interaction between tectonic uplift and climate because their sediments contain abundant information of tectonics and climate change since the Miocene (e.g., Harris, 2006; Li et al., 2014; Molnar, 2005; Y. B. Yang et al., 2016, and references therein). A series of sedimentary basins located in the northeastern TP with thick late Cenozoic sediments can provide continuously long archives to reveal the regional topographic and climatic evolution. Numerous studies have shown that widespread uplift started at approximately 8–9 Ma across the northeastern TP, as evidenced by thermochronology methods (Z. L. Chen et al., 2002; Lease et al., 2007; Zheng et al., 2003, 2006, 2010) and sedimentation rates (Fang et al., 2007, 2016; Huang et al., 2006; Li et al., 2014; R. S. Yang, Fang, et al., 2017). This series of contemporaneous uplift events should be attributed to the late tectogenesis of India‐Asia colli- sion (Tapponnier et al., 2001). Meanwhile, a nearly concurrent enhancement of aridification also occurred in the central Asian interior and manifested as intensified arid conditions and decreased precipitation amount (Guo et al., 2002; Miao et al., 2012, and references therein). The onset of eolian red clay (fine‐grained loess with reddish color) accumulation in the Chinese Loess Plateau (CLP; An et al., 2001; Ding et al., 2001; Qiang et al., 2001) and a notable increase in eolian input to the fluvial basin system on the northeastern TP (Y. B. Yang, Galy, et al., 2017), the sea of Japan (Shen et al., 2017), and the North Pacific (Rea et al., 1998; W. F. Zhang, Chen, et al., 2016) have all been interpreted as evidences of central Asian aridification; yet a new hypothesis interpreted them as a result of more dust supply caused by increased erosion in the Qilian Mountains and low‐elevation eastern Asia areas (Nie et al., 2018). Intensified arid conditions and decreased precipitation amount in the northeastern TP and its adjacent regions was also recorded by the replacement of forest by steppe (Hui et al., 2011; J. Liu, Li, et al., 2016; Ma et al., 1998, 2005), abrupt shift of geochemical proxies (Wan et al., 2010; Y. B. Yang et al., 2016), and sharp increase of illite/smectite ratio (Shen et al., 2017). However, the existing tectonic and climatic evidence is temporally simultaneous (~8–9 Ma) but spatially separated in different regions, and thus factors of local sedimentary environment and climatic conditions may bias those records, leading to poor constraints on the exact links between tectonic and climatic forcing mechanisms and processes. Furthermore, in this tectonically active region, other factors, such as changes in landform, bedrock lithology, hydrographic patterns, and sedimentary sorting, may overwhelm the paleoen- vironmental significance of proxy records (e.g., Fang et al., 2016). For example, when dealing with a long‐ term sedimentary sequence, its complex and often poorly known bedrock lithology and frequently evolving sediment routing conditions from source to sink can profoundly bias many widely used whole‐rock paleoen- vironmental proxies. Clay minerals and their assemblages are potential proxies to reconstruct paleoenviron- ment in such complex sedimentary routing systems (Singer, 1984; Thiry, 2000; Ye et al., 2018). To date, the long‐term clay mineral records that are needed to reveal the tectonic‐climate linkage during the late Cenozoic are lacking in the northeastern TP. Recently, a newly discovered fluvial‐lacustrine sedimen- tary sequence in the Xining Basin, located in the northeastern TP, was precisely dated by magnetostratigra- phy and fossil mammals and has an age range from ~12.7 to 4.8 Ma (R. S. Yang, Fang, et al., 2017). Here we present the first clay mineral and geochemical records of the clay fraction (<2 μm) to better characterize the interplay of the regional climate and tectonic uplift changes. 2. Geographic and Geological Settings The Xining Basin is situated on the northeastern edge of the TP and is a western subbasin of the larger Paleocene‐Miocene basin complex named the Longzhong Basin. The basin ranges in elevation between 2,000 and 3,000 m with surrounding mountains reaching approximately 3,700–4,000 m (Horton et al., 2004). The basin is confined by the Riyueshan Mountains, the Dabanshan Mountains, and the Lajishan Mountains to the west, north, and south, respectively, while it is open to the east to the Lanzhou Basin (Figure 1; Dai et al., 2006). Climatically, the basin is located in the transitional zone of the East Asian mon- soonal humid region and the central Asian arid region and is thus dominated by a semiarid continental cli- mate (Figure 1a; Xiao et al., 2012).